Retractable thrusters or azimuthal retractable thrusters have been long known. The azimuthal thrusters are marine thrusters rotatable about a vertical axis (hence the name of azimuthal), thereby allowing the orientation of the propeller in any horizontal directions, making the presence of a rudder redundant. These azimuthal thrusters allow a considerable maneuverability of ships, but also of floating platforms, which require to accurately maintain a position when out at sea.
Such solutions are known, for example, from U.S. Pat. No. 6,375,524, U.S. Pat. No. 6,439,936, US 2001/029133, EP 0 591 969, KR 2012/0017025, U.S. Pat. No. 6,375,524, WO 2010/136012.
Retractable thrusters are also known from the state of the art, which are retracted into housings that are suitable to the maintenance thereof.
For example, from the document U.S. Pat. No. 5,397,255 by Schottel Werft Joseph Becker GmbH. & Co., KG, a propulsion unit for a ship is known, having a thruster with a propeller that may be retracted into a cylindrical case and closed by a cover plate that is secured to the thruster, so that water present in the cylindrical case may be pumped out, once it is closed, with the purpose of ensuring the access for the thruster inspection and maintenance.
Particularly, this known solution provides for a propeller projecting under the bottom of the ship and retractable in a case arranged within the bottom, which is locally flat. Said case is surrounded at the lower edge thereof by an annular groove comprising a gasket to seal said case when the cover plate firmly at the thruster is retracted and contacts the bottom. Said plate is firmly secured in a removable manner to the thruster so that the propeller and the plate may be lifted and lowered vertically together.
This known solution, while allowing the maintenance of the thruster when it is retracted within the hull, is particularly bulky and heavy in all the steps of use thereof. Particularly, the closing plate that is firmly secured to the thruster adds additional weight, as well as a drag of the ship for the entire time of normal use of the thruster in the extended position.
Of course, the presence of this closing plate secured to the thruster makes this solution unsuitable for thrusters inclinable with respect to the vertical axis, such as, for example the solutions described in the documents U.S. Pat. No. 7,641,526 and U.S. Pat. No. 4,580,517.
Furthermore, it has to be considered that these known retractable thrusters, moreover in the case of applications on floating platforms, remain in the extended and operative position most of the time of use of the platform or ship. Therefore, the overall dimensions given by the closing plate and the ship drag thereof are a particularly felt problem.
Furthermore, in these applications, the substantially constant exposure of the closing plate to the substances that are present externally to the hull lead to a degradation of the plate portions that are suitable to the coupling with the hull, sometimes compromising the perfect coupling thereof to the hull. For example, suppose how the effect of building-up incrustations may make the closure of the plate in contact with the bottom problematic and the risks following any water leakage into the cylindrical case into which the thruster is retracted for the maintenance thereof.
Furthermore, this known solution of retractable thruster forces to the assembling of the thruster itself and the motor associated thereto on a scaffolding that allows a vertical movement suitable to the extraction and the retraction of the thruster into the hull. Therefore, this solution has necessarily to provide for the use of a bulky interspace arranged vertically in the hull, which allows, in any time during the navigation, retracting the support scaffolding of the thruster motor within the hull itself, resulting in a considerable waste of volume within the hull, which volume the ship owner would like to be intended for other paying uses.
This solution necessarily forces to a specific designing of the retractable thruster provided with the special closing plate thereof. Supposing a ship or a floating platform of medium or large dimensions, it is realized that, in order to properly maneuver, a plurality of retractable thrusters will be present, each being provided with the closing plate thereof and the scaffolding thereof for handling the thruster into the cylindrical case. The presence of this plurality of retractable thrusters, implemented according this known solution, is not only particularly heavy, but moreover bulky and arranged in different areas of the hull, thus forcing to implement several vertical interspaces, which may sometimes reach the entire height of the same hull, segmenting the internal volume of the ship, thus making the use thereof still more complex.
Other solutions are known, for example, from U.S. Pat. No. 2,987,027, U.S. Pat. No. 3,483,843, U.S. Pat. No. 2,885,990, U.S. Pat. No. 3,030,910. However, no one of these documents proposes a solution to the above-mentioned drawbacks and needs. Particularly, none of these solutions proposes a method for the maintenance of a retractable thruster assembly that is as safe and rapid as possible, while not forcing to special equipment, making so that the assembly is not very bulky and avoids any ship drag as much as possible, while ensuring at least the routine maintenance of the thruster when it is retracted within the hull.
Therefore, a particularly felt problem is to propose a method for the maintenance of a thruster assembly for a ship or a floating platform, that allows the maintenance of the thruster itself when the ship or platform is floating, minimizing the weight and the overall dimensions of the structures interlocked to the retractable thruster.